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My research focuses on understanding the nature of Dark Matter and on the exploration of the medium energy gamma-ray sky. Dark Matter is one of the great puzzles of cosmology. Even though it was first found by astronomers seven decades ago through its gravitational effects, we have uncovered some of its basic properties only more recently. Most importantly, we have learned that it constitutes the bulk of matter in the universe. A wealth of astrophysical observations indicates that the universe consists of two principal ingredients: so-called Dark Energy, an energy leading to an accelerated expansion of the universe today, and Dark Matter, the principal driver for structure formation in the universe, dominating cosmic evolution in the first few billion years after the Big Bang. On the other hand, all the familiar matter that we see around us and that we are made of, in fact all matter consisting of protons, neutrons, and electrons, constitutes a mere 4% of the content of the universe. In yet another twist to the Copernican Revolution, we are learning that we are made of rather exotic matter in a sea of much more common but invisible "dark" matter, which, however, barely interacts with the "regular" matter we are accustomed to. In other words, some 85% of the total mass in the universe remains to be discovered. But what is it? Reports by the National Research Council and other national and international science committees have recognized the puzzle of the nature of Dark Matter as one of the great puzzles in science today. As one of the founding members of the international XENON collaboration, I pursue with my research group a search for so-called Weakly Interacting Massive Particles (WIMPs) as the most promising candidate for Dark Matter in the Universe. XENON applies a novel detector technology based on liquefied xenon as detection medium to the measurement of expected elastic scatterings of WIMPs and xenon nuclei. The resulting recoil energy can be detected and distinguished from more frequent background events resulting from trace amounts of natural radioactivity within the detector or its surroundings. With our first Dark Matter detector, XENON10, operated beneath >1 km of rock at the Gran Sasso National Laboratory (LNGS) in central Italy, we provided the world’s most stringent limits on Dark Matter particle interactions with regular matter in 2007. The next step in our search for Dark Matter, XENON100, will explore the parameter space 20 times deeper, extending well into the domain of potential detections. My group is heavily involved in this project with hardware development as well as in software, simulation, and analysis efforts. XENON100 is now taking data, and we are already planing the next step to the ton-scale, XENON1T. My Dark Matter research is supported by NSF.

My second major research interest lies in the field of medium energy gamma-ray astronomy, which provides astrophysical insights that are difficult or impossible to obtain at other wavelengths. Cosmic gamma-rays are emitted by the most energetic cosmic explosions in gamma-ray bursts, by cosmic accelerators in the jets of mass-accreting supermassive or stellar black holes, by rotating magnetized neutron stars, or by cosmic-ray interactions with the interstellar medium. Nuclear line fingerprints of radioisotopes and the matter-antimatter (electron-positron) annihilation line are produced in various astrophysical environments, in particular in supernova explosions. Imaging spectroscopy of these lines provides deep insight into their nucleosynthesis and probes the physics of these sources. Observing the gamma-ray sky in the energy band of nuclear transitions is as challenging as it is promising. Even though gamma-rays can pass the entire Milky Way galaxy without being absorbed, Earth's atmosphere is opaque to this radiation. Consequently, we must put our telescopes on satellites or high altitude balloons. In space, cosmic rays and trapped particles in Earth's radiation belts constantly bombard the detector as well as any support structures, leading to a bright glow of gamma-rays just in the energy range to be observed. At the same time, fluxes from astrophysical sources are comparatively weak. Gamma-rays cannot easily be focused as is done, e.g., with telescopes at optical wavelengths. As a consequence, the signal/background ratio is relatively low. These challenges require large gamma-ray detectors with fine position resolution, good energy resolution, and means of background suppression. The detector technology my group is developing, with NASA funding, is based on modules of so-called Liquid Xenon Time Projection Chambers, which are a promising technology for a future satellite mission in gamma-ray astronomy.